The present invention relates to an image forming apparatus arranged to bring a charger, such as a charging roller, into contact with a photosensitive member to charge the photosensitive member and form an electrostatic latent image on the charged photosensitive member so as to form an image. More particularly, the present invention relates to an image forming apparatus incorporating a cleaner for cleaning a charger and arranged to remove foreign matter, such as toner, allowed to adhere to the charger.
An image forming apparatus, such as an electrostatic copying machine and a printer, incorporates a charging unit for charging a photosensitive member. Hitherto, corona charge has widely been adopted to the charging unit. The corona charge, however, suffers from a problem of a defect of a formed image because ozone or nitrogen oxide is formed which adheres to the surface of the photosensitive member or the like.
Therefore, a charger apparatus adapted to a contact charging method has been disclosed in, for example, Japanese Patent Publication No. 63-49668A. The foregoing method is arranged to bring a charger to which voltage is applied and which is constituted by, for example, urethane rubber into contact with the photosensitive member so as to charge the photosensitive member.
The contact charging method is arranged such that the charger is in contact with the photosensitive member. Therefore, the contact charging method encounters adhesion of foreign matter to the surface of the charger, the foreign matter being, for example, toner on the photosensitive member, undesirably allowed to pass through the cleaning unit or toner separated from the developing unit. Therefore, defective charge of the photosensitive member occurs, causing the quality of the formed image to deteriorate.
Therefore, pieces of image forming apparatus of a type incorporating a cleaner for removing foreign matter, such as toner, allowed to adhere to the surface of the charger have been suggested. When the cleaner of the image forming apparatus is always in contact with the charger, the surface of the charger sustains damage. In the foregoing case, defective charging occurs. Therefore, the cleaner is disposed such that contact and separation with respect to the charger are permitted. The cleaner is brought into contact with the charger only when the charger is cleaned. When the charger is not cleaned, the cleaner is separated from the charger. Usually, the charger is periodically cleaned when a printing operation is not performed. For example, in Japanese Patent Publication No. 7-128954A, the charger is cleaned whenever a predetermined number of image forming operations has been performed. In Japanese Patent Publication No. 7-128956A, the charger is cleaned at intervals of predetermined time.
A variety of pieces of image forming apparatus have been suggested in, for example, Japanese Patent No. 2853208. The foregoing image forming apparatus has a structure that the photosensitive member, the development unit and the cleaning unit are accommodated in a single process cartridge. The process cartridge is detachably mounted on a predetermined position in the body of the image forming apparatus. When the photosensitive member or the like deteriorates, the process cartridge is changed.
The related image forming apparatus incorporating the member for cleaning the charger and arranged to use a detachable process cartridge has a structure that the cleaner is provided for the body of the image forming apparatus without exception.
When the cleaner is provided for the body of the image forming apparatus, (1) since the cleaner is provided for the purpose of removing foreign matter, such as toner, allowed to adhere to the charger, cleaning conditions including the contact load at which the cleaner is brought into contact with the charger and the position of contact must be optimized. Since the cleaner is provided for the body of the image forming apparatus, optimization of the cleaning conditions cannot easily be performed because cleaning conditions are undesirably varied. As a result, the life of the charger is shortened undesirably. When the contact load is too large, the surface of the charger sustains damage. When the contact load is too small, the performance for removing foreign matter, such as toner, deteriorates.
(2) Since the cleaning unit must be changed individually from change of the process cartridge, a user must bear a heavy load, such as labor for changing the cleaning unit.
Besides, when foreign matter, such as toner, adheres to the charger, the foregoing deterioration in the image does not occur. That is, the density of the output image is sometimes changed. In the foregoing case, the charger is not cleaned with the cleaner. That is, the density of the image is adjusted to correct the change in the density of the image. The correction is realized by patch control (charging bias or a development bias is adjusted in the foregoing case).
The patch control is, as disclosed in, for example, Japanese Patent Publication No. 7-111591B, performed to correct change in the density of an image in a case where the density of the image is changed according to change in the use environment and length of the operation time.
When change in the density of an image occurring when foreign matter, such as toner, adheres to the charger is corrected by the foregoing patch control, an operation for cleaning the charger which is performed after the patch control has been performed sometimes results in change in the density of the image in spite of completion of the adjustment of the density of the image by performing the patch control.
Besides, a cleaning unit incorporating a mechanism for permitting contact/separation of the cleaner and arranged to clean the charger has a structure that toner removed from the charger by the cleaner is accumulated in a nip portion in which the charger and the cleaner are in contact with each other when the cleaner is separated from the charger. A portion of toner is moved to the photosensitive member. Therefore, when the operation of the photosensitive member is interrupted at arbitrary timing, removed toner is sometimes left on the photosensitive member. When the image forming apparatus has started a printing operation in the foregoing state, movement of toner removed and left on the photosensitive member to the transferring position undesirably causes toner to be transferred. Thus, there arises a problem in that an image defect occurs.
Therefore, when the cleaner periodically cleans the charger, occurrence of an image defect caused from transference of toner removed and left on the photosensitive member must be prevented.
In general, an electrophotographic method is arranged to bring a rubber blade into contact with a photosensitive member under a predetermined pressure so as to remove residual toner on the photosensitive member after an image has been transferred therefrom. Abrasion between the photosensitive member and the rubber blade causes the leading end of the blade to repeat small mechanical vibration during the operation. In cases where the vibrations are amplified, a defect state of cleaning occurs momentarily. Thus, residual toner is sometimes leaked downstream of the blade. Thus, toner adheres to the charging roller, causing contamination to occur.
Hitherto, the contamination of the charging roller has been prevented by the following suggested methods:
(1) a method with which a cleaner is pressed against the charging roller to mechanically remove contamination (for example, Japanese Patent Publication No. 6-342237A); and a method with which a cleaner is vibrated in the axial direction of a charging roller to improve cleaning efficiency (for example, Japanese Patent Publication No. 7-110618A); PA1 (2) a method with which toner allowed to adhere to the charging roller is uniformed to prevent occurrence of line-shape contamination which causes a problem image (for example, Japanese Patent Publication No. 7-168422A).
Since the charging roller is constituted by resin, rubber or the like, the foregoing method (1) with which contamination is mechanically removed requires a structure that a pad, rubber, sponge or the like is employed as the cleaner to remove contamination. Therefore, it is very difficult to remove toner allowed to adhere the surface of the charging roller. Although toner can be removed when the cleaner is strongly pressed against the charging roller, the surface of the roller easily sustains damage. When the cleaner is pressed with a low pressure to prevent damage of the surface of the roller, toner is undesirably left in the form of lines on the surface of the charging roller. Therefore, the contact pressure between the cleaner and the charging roller cannot easily be adjusted.
The foregoing method (2) with which adhesive toner is uniformed is a method of removing toner. As the apparatus is used, contamination is gradually accumulated. Thus, the charging roller is covered with toner. Therefore, a radical countermeasure against contamination cannot be realized.